Sirolimus and Post Transplant Cyclophosphamide (PT-Cy) Allow the Use of Haploidentical PBSC Grafts Inducing a Favorable Immune Reconstitution with Low Rates of GvHD: Results in 39 Patients

Introduction:Haploidentical hematopoietic stem cell transplantation (HSCT) performed using T–replete BM grafts and post-transplantation cyclophosphamide (PT-Cy) has gained much interest in the transplantation community for the excellent toxicity profile, achieving outcomes equivalent to those of HSCT performed using matched related or unrelated donors (Bashey, JCO 2013). Nonetheless, relapse remains a major challenge with the use of this non-myeloablative regimen (Luznik, BBMT 2008). Thus, we investigated whether the use of PBSC graft, an intensified myeloablative conditioning and sirolimus instead of tacrolimus might help increasing the therapeutic index of this procedure.

Patients and Methods:Here we report the results on 39 patients treated between Nov 2012 and June 2014. The median age was 56 years (range, 21–78). Diagnoses included AML (21), ALL (5), CML-BC (1), MDS (1), NHL (5), and HD (6). The majority (62%) of patients were not in disease remission at time of HSCT. Eight patients (20%) received a prior allo-HSCT. For the remaining patients who underwent haplo-PTCy as a first allogeneic transplant, 19 patients (61%) scored high/very high according to the CIBMTR disease score, 12 (39%) scored intermediate, while no patient had a low disease score. The median Sorror Comorbidity Index was 3 (range: 0-9). Conditioning consisted of treosulfan (14 g/m2/day) on days –6 to –4, fludarabine (30 mg/m2/day) on days –6 to –2, and melphalan (70 mg/m2/day) on days –2 and –1, followed by T-replete G-CSF-mobilized PBSC. Postgrafting immunosuppression consisted of PT-Cy (50 mg/kg/day) on days 3 and 4, followed by mycophenolate mofetil for 30 days, and sirolimus for at least 3 months. Overall survival (OS) and disease-free survival (DFS) were estimated using the Kaplan-Meier method. Cumulative incidence (CI) of non-relapse mortality (NRM) was calculated using relapse as a competing risk, for GvHD the competing risk was death.

Results:PBSC grafts contained an infused median CD34/kg of 6.03 e6 (4.15–8.02 e7), CD3/kg 21.12 e7 (10.5–48.26 e7). All patients but 1 engrafted (97.5%), with a median time to neutrophil and platelet recovery of 18 days (range 13–45) and 19 days (9–65), respectively. Notably, the only patient with primary graft failure had a HLA-DP mismatch in HvG direction. Post-HSCT recovery of lymphocyte subsets was broad and fast: median time to CD3>100/ml was 28 days (range: 8-97), to CD4>200/ml 35 days (21-137) and to CD19>0/ml 34 days (24-137). Circulating T cells comprised naïve and memory subsets, with a recovery of CD31+ recent thymic emigrants (RTEs) starting from day 30. All patients had a significantly higher proportion of circulating RTEs at day 30, 90 and 180 compared to their pre-HSCT levels, suggesting an improvement in their thymic function after HSCT. We observed a significant early increase in circulating Tregs at day 15 post HSCT.

With a median follow-up for living patients of 327 days, the 1-year cumulative CI of NRM and relapse were 15% and 32%, respectively. Two of the 9 relapses were HLA-loss variants (Vago, NEJM 2009) occurring at a median time of 277 days post-HSCT. Classical relapses had instead an earlier occurrence (median time to relapse: 92 days, range 37-134). CI of acute GvHD grade II-IV and III-IV at 6 months were 18% and 8%, while CI of chronic GvHD at 12 months was 19%.

Estimated 1-year probability of OS and DFS were 58% and 51%. Outcomes were particularly favorable in patients transplanted in CR (1-year OS 93%, DFS 95%) as compared to those of patients transplanted with active disease (1-year OS 37%, DFS 29%; p=0.005 and p=0.001, respectively). Importantly, stratification according to CIBMTR score held similar results: in patients with intermediate score, 1-year OS was 91% vs. 33% in patients with high/very high score (p=0.015); similarly, 1-year DFS was 92% vs. 34% (p=0.009).

Conclusions: PT-Cy coupled to sirolimus allow the use of haploidentical PBSC grafts with low rates of GvHD, and intensified myeloablative regimen is a valid option for patients with aggressive/advanced hematologic malignancies. The acceptable rates of GvHD and NRM as well as the favorable immune reconstitution profile open the way for combining it with novel immunomodulatory or cellular therapies to improve DFS in patients at high risk for relapse who would benefit from further treatment intensification early after transplant.